By having the PRE-AF on, the camera continuously focuses. As long as the power switch is on, it will focus. So you do not need to press the shutter button halfway down for focusing.
The PRE-AF speeds up the lag in the focusing of the time it takes to half-press the shutter button.
Menu > AF/MF setting tab > PRE-AF > ON | OFF
The PRE-AF uses about 40% more power than not having it on. So from 250 shots per battery charge, we go down to 150 shots per battery charge.
And that’s for Single Shot, one shot every 30 seconds. In the real world, we are getting a lot closer to 100 single shots per battery.
I have spent many months having it on, and many months having it off.
I have decided that the downside of so much less battery power doesn’t warrant its use for me. Now an X-T2 with the optional battery grip, that would be another story. We’d be talking between 300 and 400 shots per battery pack recharge…